This application claims the benefit of Japanese Patent application No. 2000-137730 which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a progressive power multifocal lens, and more specifically, to a progressive power multifocal lens which is used to assist an accommodation power of an eye.
2. Related Background Art
In order to correct presbyopic eyes, a single focal lens, a bifocal lens, a progressive power multifocal lens, and the like, are conventionally used. Especially with respect to a progressive power multifocal lens, out of these lenses, the glasses which use the progressive power multifocal lenses are not required to be replaced or removed for looking at a distant point and for looking at a near point. For the outward looks, a progressive power multifocal lens has no border line, unlike in the case of a bifocal lens. As a result, a demand on progressive multifocal lenses is considerably increasing recently.
A progressive multifocal lens is a spectacle lens to assist an accommodation power of an eye when the accommodation power of the eye becomes weak so that a near vision is difficult to see clearly. In general, a progressive multifocal lens has a far vision correction portion which is positioned in an upper part of the lens when the lens is worn (hereinafter called xe2x80x9cthe far portionxe2x80x9d), a near vision correction portion which is positioned in an lower part of the lens when the lens is worn (hereinafter called xe2x80x9cthe near portionxe2x80x9d), and a progressive power portion in which the refracting power is continuously changed between the both portions (hereinafter called xe2x80x9cthe intermediate portionxe2x80x9d). Note that the terms xe2x80x9cupperxe2x80x9d, xe2x80x9clowerxe2x80x9d, xe2x80x9chorizontalxe2x80x9d and xe2x80x9cverticalxe2x80x9d in the present invention are used to refer to positional relations in the lens when it is worn. For instance, the lower part of the far portion indicates an area which is inside the far portion and close to the intermediate portion.
FIG. 1 is a view for showing an outline of the divided regions of a progressive power multifocal lens which is designed symmetrically. The progressive power multifocal lens shown in FIG. 1 comprises a far portion F which is located in an upper part of the lens when it is worn, a near portion N located in a lower part, and an intermediate portion P located between the far portion and the near portion in which the refracting power is continuously changed. As for the shape of a lens surface, an intersecting line MMxe2x80x2 of a cross section along the meridian passing through approximately the center of the lens surface and extending vertically from top to bottom and the lens surface on the object side (the side opposite to the eyes) is used as a reference line for expressing lens specifications such as the addition power of the lens and is also used as a significant reference line in the lens design. In the progressive power multifocal lens thus symmetrically designed, the center OF of far portion of the far portion F, an eyepoint E serving as a fitting point, the geometric center OG of the lens surface, and the center ON of the near portion are located on the center line MMxe2x80x2 serving as the reference line.
FIG. 2 is a view for showing an outline of the divided portions of a progressive power multifocal lens in which the near portion N is asymmetrically located in consideration of the fact that the center of the near portion ON comes close to the nasal side when the lens is worn (hereinafter called an xe2x80x9casymmetrical type progressive power multifocal lensxe2x80x9d). Also in such an asymmetrical type progressive power multifocal lens as shown in FIG. 2, the center line MMxe2x80x2 which comprises the intersecting line of the cross section which passes through the center of distance vision portion OF of the far portion F, the eyepoint E for the distance portion, the geometric center OG of the lens surface, and the center of near vision portion ON and the object-side lens surface is used as the reference line.
In the present invention, these reference lines are collectively called the xe2x80x9cprincipal meridianxe2x80x9d. The center of the far portion F and the center of the near portion N are used as the reference positions for measuring lens powers. The reference point for measuring the lens power of a far portion is called the center of distance vision portion OF, while the reference point for measuring the lens power of a near portion is called the center of near vision portion ON. Furthermore, the mean surface refracting power of the center of distance vision portion OF is defined as a base curve, and the mean dioptric power of a transmitting light beam which passes through the center of distance vision portion OF is defined as the reference mean dioptric power in the far portion (hereinafter called xe2x80x9cthe distance dioptric powerxe2x80x9d). Normally, the center of the near portion is coincident with the eyepoint for the near vision. The terms xe2x80x9cthe center of distance vision portionxe2x80x9d or xe2x80x9cthe center of near vision portionxe2x80x9d does not indicate the geometric center in each of the portions, but indicates the functional center when the lens power is measured or the lens is worn.
According to the present invention, the mean surface refracting power (hereinafter called xe2x80x9cthe refracting powerxe2x80x9d) and the surface astigmatism (hereinafter called xe2x80x9cthe astigmatismxe2x80x9d) can be respectively expressed by the following equations (a) and (b), where the maximum main curvature is xcfx86max and the minimum main curvature is xcfx86min at an arbitrary point on the progressive power multifocal surface:
the refracting power=(xcfx86max+xcfx86min)xc3x97(nxe2x88x921)/2xe2x80x83xe2x80x83(a);
and
the astigmatism=(xcfx86maxxe2x88x92xcfx86min)xc3x97(nxe2x88x921)xe2x80x83xe2x80x83(b).
Also according to the present invention, the mean dioptric power and the astigmatism can be respectively expressed by the following equations (c) and (d), where the maximum dioptric power and the minimum dioptric power in a light beam transmitted through an arbitrary point on the progressive power multifocal surface are defined as Dmax and Dmin, respectively:
the dioptric power=(Dmax+Dmin)/2xe2x80x83xe2x80x83(c);
and
the astigmatism=(Dmaxxe2x88x92Dmin)xe2x80x83xe2x80x83(d).
Furthermore, according to the present invention, the mean surface additional refracting power (hereinafter called xe2x80x9cthe surface additional refracting powerxe2x80x9d) is a refracting power which is obtained by subtracting the base curve from the refracting power at an arbitrary point on the progressive power multifocal surface. On the other hand, the mean additional dioptric power (hereinafter called the xe2x80x9cadditional dioptric powerxe2x80x9d) is a dioptric power which is obtained by subtracting the distance dioptric power from the mean dioptric power (hereinafter called the xe2x80x9cdioptric powerxe2x80x9d) of a light beam passing through an arbitrary point on the progressive power multifocal surface power.
It is noted that, in the progressive power multifocal lens, a positive refracting power (or dioptric power) is continuously imparted from the center of distance vision portion OF toward the center of near vision portion ON on the principal meridian MMxe2x80x2 which approximately passes through the geometric center of the lens, and a value which is obtained by subtracting the refracting power (or dioptric power) of the center of distance vision portion OF from the additional refracting power (or additional dioptric power) of the center of near vision portion ON at which this addition refracting power (or additional dioptric power) approximately reaches the maximum is called the additional power of the progressive power multifocal lens. A progressive power multifocal lens which has wide clear vision ranges of all of the far vision portion F, the intermediate portion P, and the near portion N are wide, small fluctuation, distortion, and the like of an image, and is easily worn is an ideal progressive power multifocal lens.
Incidentally, as for a conventional progressive power multifocal lens, generally the optical characteristics of the progressive power multifocal surface (refracting surface) thereof were mainly discussed. That is, the performance of a progressive power multifocal lens was often evaluated in terms of a distribution of the refracting power (or a distribution of the additional refracting power), a distribution of astigmatism, or the like, of the progressive power multifocal surface. For this reason, the lens designer mainly aimed of obtaining a distribution of the refracting power which is suitable for a use of the progressive power multifocal lens, securely forming a wide space for a so-called a clear vision range which has astigmatism of not more than a predetermined value, and suppressing the maximum value of the astigmatism to the minimum, taking swim, fluctuation or distortion of an image which may be caused upon movement of the eye into consideration.
However, in an actual spectacle lens, the optical characteristics of the progressive power multifocal surface of the lens are not necessarily coincident with the optical characteristics of the lens when it is actually worn by the wearer. For this reason, recently, in order to improve the optical characteristics of the lens when the wearer actually wears the lens, not only the optical characteristics of the progressive power multifocal surface, but also the optical performance of the lens in a state which is closer to the worn state come to be evaluated. That is, the optical performance of the lens comes to be evaluated by a light beam transmitted through the lens.
In general, the relation between the lens curvature and the lens power to make the astigmatism of a light beam transmitted through the lens to be the minimum can be obtained from, for instance, a Tscherning""s Ellipse. That is, it is well known that generation of astigmatism in the peripheral area of the lens can be suppressed by selecting an optimal combination of curvatures which can be obtained from this Tscherning""s Ellipse. However, when the optimal combination of curvatures obtained from this Tscherning""s Ellipse is used, there is a tendency that the curvature of the base curve becomes large and the lens thickness also becomes large. As a result, with respect to progressive power multifocal lenses recently manufactured, for reducing the thickness of the lens or improving the external appearance, or for a reason for manufacturing, a curvature smaller than that obtained by such an optimal combination of curvatures is mainly selected as the base curve.
Accordingly, a distribution of the refracting power or a distribution of the astigmatism on the progressive power multifocal surface have the same tendency with a distribution of the dioptric power or a distribution of the astigmatism of a light which passes through the lens to enter an eye of the wearer of the lens limitedly in most cases in an area in which a light from an object enters the lens surface at an approximately right angle, that is, an area in the vicinity of the optical axis of the lens, such as, an area around the fitting point of the lens. On the other hand, a light entering an eye of the wearer through a position separated from the optical axis of the lens is incident on the lens surface in a slanting manner. Thus, astigmatism is generated even when a light passing through a position at which astigmatism on the lens surface is substantially zero passes through the lens, and this light is to enter an eye of the wearer in a state in which the dioptric power deviates from the dioptric power for distance vision which serves as the reference dioptric power. This tendency is different depending on the curvature of the lens on the prescription side or the thickness of the central portion thereof, and becomes larger toward the periphery of the lens.
More specifically, in a progressive power multifocal lens having a plurality of base curves, when a distribution of the refracting power and that of astigmatism of the progressive power multifocal surface are designed to be equal with respect to the base curves, the distribution of dioptric power and the distribution of astigmatism of a transmitted light beam are substantially different depending on each of the base curves. As a result, in order to obtain a series of progressive power multifocal lenses having a plurality of base curves in which optical characteristics, such as a distribution of the additional dioptric power or a distribution of astigmatism, of a transmitted light in the worn state are equal with respect to the plurality of base curves, such a design is required which optimizes the progressive power multifocal surface taking into consideration a manufacture range of each of the base curves.
Recently, such conventional technologies in which the optical performance by these transmitted lights is evaluated in a progressive power multifocal lens are proposed. However, according to these conventional technologies, a portion having an astigmatism in a predetermined amount or less, more specifically an astigmatism of 0.50 diopters or less, is defined as a clear vision range, and it is discussed in most cases only to secure this clear vision range to be wide. That is, according to the conventional technologies, optimization of the dioptric power distribution is scarcely discussed. Furthermore, a technology for optimizing a far portion as a surface to conform to each of the base curves having different curvatures has not been yet proposed.
To suppress the astigmatism to the minimum is significant and essential in order to widen the clear vision range in the worn state. However, particularly with respect to the far portion, it is not sufficient to define the clear vision range only in terms of an amount of the astigmatism. That is, in a portion in which the dioptric power largely deviates from the distance dioptric power which is predetermined by the prescription, even if the astigmatism is in an amount less than a predetermined value which is defined for the clear vision range, a blur of the image is generated due to this dioptric power error, so that the wearer can not see an object distinctly in a distance vision. An influence of the dioptric power error in the far portion for distance vision is greater than that in the near portion for near vision. For this reason, it is more important in the far vision portion to design the lens taking into consideration a dioptric power error from a predetermined distance dioptric power, rather than in the near portion.
The present invention has been contrived taking the above-described problems into consideration, and has its object to provide a series of progressive power multifocal lenses having a plurality of base curves which are designed to have substantially the same basic lens specifications and substantially the same optical characteristics in the worn state with respect to all of the base curves, so as to have an excellent optical performance in the worn state. The present invention particularly has its object to provide a progressive power multifocal lens which can secure a wide clear vision range with a smaller amount of astigmatism in the far portion and a smaller amount of blur of the image due to a dioptric power error.
In order to solve the above problems, according to a first invention, there is provided a series of progressive power multifocal lenses which have a plurality of base curves designed to have substantially the same basic lens specifications, each lens having a far vision correction portion corresponding to a distant vision, a near vision correction portion corresponding to a near vision, and a progressive portion for continuously connecting the refracting powers of the surfaces of the both portions between the far vision correction portion and the near vision correction portion along the principal meridian for dividing the refracting surface of each lens into the nasal side portion and the temporal side portion, characterized in that:
the following condition (1) is satisfied at least in either one of the nasal side portion and the temporal side portion with respect to the principal meridian which satisfies 15xe2x89xa6|x|xe2x89xa620:
xcex94PS(x,0) greater than xcex94PL(x,0)xe2x80x83xe2x80x83(1),
Where:
in a first progressive power multifocal lens having a first base curve BCL selected from the plurality of base curves, the mean surface refracting power of a point on a lens refracting surface which is separated by x (mm) in the horizontal direction from an eyepoint for a distance vision in a state in which the lens is worn is defined as PL(x,0) (diopter), and the mean surface additional refracting power which is obtained by subtracting the first base curve BCL from the mean surface refracting power is defined as xcex94PL(x,0) {=PL(x,0)xe2x88x92BCL} (diopter): and
in a second progressive power multifocal lens having a second base curve BCS which has a substantially smaller curvature than the first base curve BCL and selected from the plurality of base curves and having substantially the same additional power as the additional power of the first progressive multifocal lens, the mean surface refracting power of a point on a lens refracting surface which is separated by x (mm) in the horizontal direction from the eyepoint in a state in which the lens is worn as PS(x,0) (diopter), and the mean surface additional refracting power which is obtained by subtracting the second base curve BCS from the mean surface refracting power is defined as xcex94PS(x,0) {=PS(x,0)xe2x88x92BCS} (diopter).
According to a second invention, there is provided a series of progressive power multifocal lenses which have a plurality of base curves designed to have substantially the same basic lens specifications, each lens having a far vision correction portion corresponding to a distant vision, a near vision correction portion corresponding to a near vision, and a progressive portion for continuously connecting the refracting powers of the surfaces of the both portions between the far vision correction portion and the near vision correction portion along the principal meridian for dividing the refracting surface of each lens into the nasal side portion and the temporal side portion characterized in that:
the following condition (2) is satisfied at least in either one of the nasal side portion and the temporal side portion with respect to the principal meridian which satisfies 15xe2x89xa6(x2+h2)xc2xdxe2x89xa620:
xcex94PS(x,h) greater than xcex94PL(x,h)xe2x80x83xe2x80x83(2),
Where:
in a first progressive power multifocal lens having a first base curve BCL selected from the plurality of base curves, the mean surface refracting power of a point on a lens refracting surface which is separated by x (mm) in the horizontal direction in a state in which the lens is worn from the center of distance vision portion which is separated by h (mm) from the eyepoint for a distance vision in the vertical direction in a state in which the lens is worn is defined as PL(x,h) (diopter), and the mean surface additional refracting power which is obtained by subtracting the first base curve BCL from the mean surface refracting power PL(x,h) is defined as xcex94PL(x,h) {=PL(x,h)xe2x88x92BCL} (diopter): and
in a second progressive power multifocal lens having a second base curve BCS which has a substantially smaller curvature than the first base curve BCL and selected from the plurality of base curves and having substantially the same additional power as the additional power of the first progressive multifocal lens, the mean surface refracting power of a point on a lens refracting surface which is separated by x (mm) in the horizontal direction in a state in which the lens is worn is defined as PS(x,h) (diopter), and the mean surface additional refracting power which is obtained by subtracting the second base curve BCS from the mean surface refracting power is defined as xcex94PS(x,h) {=PS(x,h)xe2x88x92BCS} (diopter).
According to a third invention, there is provided a series of progressive power multifocal lenses which have a plurality of base curves designed to have substantially the same basic lens specifications, each lens having a far vision correction portion corresponding to a distant vision, a near vision correction portion corresponding to a near vision, and a progressive portion for continuously connecting the refracting powers of the surfaces of the both portions between the far vision correction portion and the near vision correction portion along the principal meridian for dividing the refracting surface of each lens into the nasal side portion and the temporal side portion, characterized in that:
the following condition (3) is satisfied at least in either one portion of the nasal side portion and the temporal side portion with respect to the principal meridian which satisfies 15xe2x89xa6(x2+y2)xc2xdxe2x89xa620:
xcex94PS(x,y) greater than xcex94PL(x,y)xe2x80x83xe2x80x83(3),
Where:
in a first progressive power multifocal lens having a first base curve BCL selected from the plurality of base curves, the mean surface refracting power of a point on a lens refracting surface which is separated by x (mm) in the horizontal direction in a state in which the lens is worn from the eyepoint for a distance vision and which is separated by y (mm) in the vertical direction from the eyepoint in a state in which the lens is worn is defined as PL(x,h) (diopter), and the mean surface additional refracting power which is obtained by subtracting the first base curve BCL from the mean surface refracting power is defined as xcex94PL(x,y) {=PL(x,y)xe2x88x92BCL} (diopter): and
in a second progressive power multifocal lens having a second base curve BCS which has a substantially smaller curvature than the first base curve BCL and selected from the plurality of base curves and having substantially the same additional power as the additional power of the first progressive multifocal lens, the mean surface refracting power of an arbitrary point on a lens refracting surface which is separated by x (mm) in the horizontal direction in a state in which the lens is worn from the eyepoint and which is separated by y (mm) from the eyepoint in the vertical direction in a state in which the lens is worn is defined as PS(x,y) (diopter), and the mean surface additional refracting power which is obtained by subtracting the second base curve BCS from the mean surface refracting power is defined as xcex94PS(x,y) {=PS(x,y)xe2x88x92BCS} (diopter).
According to a preferred aspect of the third invention, the following condition (4) may be satisfied in at least one of the far vision correction portion on the nasal side and that on the temporal side with respect to the principal meridian which satisfies 15xe2x89xa6(x2+y2)xc2xdxe2x89xa620:
xe2x88x920.850xe2x89xa6(xcex94Pl(x,y)xe2x88x92xcex94PS(x,y)/(BCLxe2x88x92BCS)xe2x89xa6xe2x88x920.010xe2x80x83xe2x80x83(4).
According to a first preferred aspect of the first to third inventions, the following condition (5) may be satisfied at least in either one of the nasal side portion and the temporal side portion with respect to the principal meridian which satisfies 15xe2x89xa6|x|xe2x89xa620:
CL(x,0) greater than CS(x,0)xe2x80x83xe2x80x83(5),
Where:
in the first progressive multifocal lens, the surface astigmatism of a point on a lens refracting surface which is separated from the eyepoint by x (mm) in the horizontal direction in a state in which the lens is worn is defined as CL(x,0) (diopter); and
in the second progressive multifocal lens, the surface astigmatism of a point on a lens refracting surface which is separated from the eyepoint by x (mm) in the horizontal direction in a state in which the lens is worn is defined as CS(x,0) (diopter).
According to a second preferred aspect of the first to third inventions, the following condition (6) may be satisfied at least in either one of the nasal side portion and the temporal side portion with respect to the principal meridian which satisfies 15xe2x89xa6(x2+h2)xc2xdxe2x89xa620:
CL(x,h) greater than CS(x,h)xe2x80x83xe2x80x83(6),
Where:
in the first progressive multifocal lens, the surface astigmatism of a point on a lens refracting surface which is separated by x (mm) in the horizontal direction in a state in which the lens is worn from the center of distance vision portion which is separated from the eyepoint by h (mm) in the vertical direction in a state in which the lens is worn is defined as CL(x,h) (diopter); and
in the second progressive multifocal lens, the surface astigmatism of a point on a lens refracting surface which is separated from the center of distance vision portion by x (mm) in the horizontal direction in a state in which the lens is worn is defined as CS(x,h) (diopter).
According to a third preferred aspect of the first to third inventions, the following condition (7) may be satisfied at least in either one of the nasal side portion and the temporal side portion with respect to the principal meridian which satisfies 0xe2x89xa6yxe2x89xa6h and 15xe2x89xa6(x2+y2)xc2xdxe2x89xa620:
CL(x,y) greater than CS(x,y)xe2x80x83xe2x80x83(7),
Where:
a distance between the eyepoint and the center of distance vision portion in a state in the vertical direction in which the lens is worn is defined as h (mm);
in the first progressive multifocal lens, the surface astigmatism of an arbitrary point on a lens refracting surface which is separated by x (mm) in the horizontal direction in a state in which the lens is worn from the eyepoint and is separated by y (mm) in the vertical direction in a state in which the lens is worn from the eyepoint is defined as CL(x,y) (diopter); and
in the second progressive multifocal lens, the surface astigmatism of an arbitrary point on a lens refracting surface which is separated by x (mm) in the horizontal direction in the lens wearing state from the eyepoint and is separated by y (mm) in the vertical direction in a state in which the lens is worn from the eyepoint is defined as CS(x,y) (diopter).
According to a third preferred aspect of the first to third inventions, the following condition (8) may be satisfied at least in either one of the nasal side portion and the temporal side portion with respect to the principal meridian which satisfies 0xe2x89xa6yxe2x89xa6h and 15xe2x89xa6(x2+y2)xc2xdxe2x89xa620:
0.010xe2x89xa6(CL(x,y)xe2x88x92CS(x,y))/(BCLxe2x88x92BCS)xe2x89xa60.900xe2x80x83xe2x80x83(8).